Police reports from last year show an apparent pattern of sexual harassment by students at a Catholic all-boys high school in San Mateo, prompting a teacher to file a lawsuit in San Francisco Superior Court Thursday alleging students took inappropriate “up-skirt” photos of her and other female teachers.

Kimberly Bohnert alleges last May students at Junipero Serra High School, located at 451 W. 20th Ave., had taken “up-skirt” photographs and graphic video of her that was spread around the student body.

Bohnert, who began working for the Archdiocese at Serra High School in 2006, says in the suit she was “forced” to take an extended leave of absence last year because of the harassment.

She claims students had similarly targeted other female teachers in previous years in incidents dating back at least to 2007 but the school never looked further into the allegations.

In one incident in February 2012 where sexually explicit graffiti about her was written on a boys’ bathroom stall she claims staff told her to “let it go” and that “boys will be boys.”

It wasn’t until November 2012, she claims, that two boys were suspended after posting sexually explicit tweets about her.

Bohnert is represented by Berkeley attorney Deborah Kochan.

In a news release Friday, Bohnert’s spokeswoman Edit Ruano said Bohnert had “experienced extreme incidents of sexual harassment from students and complicity from administrators who failed to take appropriate action to stop this pattern of abuse.”

Bohnert claims the school took no action against the students in the May 2013 incident until she told administrators she was taking the case to police.

The lawsuit alleges that at that point the school told San Mateo police about the photographs and named certain boys that Bohnert had confronted and believed were responsible for spreading the images.

According to police reports from the start of the investigation on May 17, 2013, a student involved in taking the photo and sharing it with classmate told investigators it was not his idea, but that he had done it.

He said the practice of taking “under the skirt” pictures of female teachers at Serra had been going on for more than three years. According to the student, a junior, that when he arrived at the school as a freshman in 2011 the practice was already in place and it was considered a “challenge.”

Apparently there are “major bragging rights” for anyone who can take the illicit photos and that students become instantly popular and reach “infamous status,” according to the report.

The same student said there were up-skirt photos of two other female teachers circulating at the time, which he heard about when another student was bragging about taking the shots.

The student said he had the opportunity to take the photo during a class on May 13, 2013, when a group of students had decided that whichever student had the best shot would take the picture.

He said he asked the teacher a question to distract her and was able to take the photo without detection, according to the report.

According to San Mateo police, school staff fully cooperated in the investigation to determine which students had been taking and spreading the inappropriate photos and videos.

Investigators found six students most responsible for producing and distributing the images, according to police.

The six students were deferred to the San Mateo County Juvenile Probation Department.

Following the investigation, police and school faculty held meetings with students and staff and warned them about these types of incidents and the legal ramifications, according to police.

There have been more trainings since the incident was reported last year, police said.

Serra High School president Lars Lund released a revised statement about the allegations Friday that said after the investigation last year the students involved were expelled.

“This was an unconscionable act, and they are no longer students at Serra,” he said in the statement.

According to the statement, Bohnert signed a contract on March 19, 2014, in which she agreed to return to Serra in the fall.

The statement said the schools is “mystified that she has signed the contract and would take this action,” referring to the lawsuit against the school filed Thursday.

“The past incidents ... were promptly and professionally addressed by Serra administrators. We have worked hard at Serra to create a safe, nurturing, educational environment according to the direction of best practices and continue to do so,” he said in the statement.

He said the school could not go into details about the suit, but he gave a detailed account of the actions the school took after Bohnert informed them about the inappropriate footage last May.

He said on May 16, the schools started “an aggressive and thorough investigation” and reported the matter to police the next morning.

The school allowed police to interview students and fully cooperated with the investigation and ultimately expelled the six students “responsible for these deeply offensive acts,” the school president wrote.

Additionally, six other students were suspended for forwarding the photos on their cellphones, he said.

According to Lund, “the immediate shock over this turn of events quickly turned to outrage, and a desire by the entire Serra community to bring those responsible to justice.”

He said teachers were informed and the parents community was sent an email on May 17. Follow-up letters were sent out over the next several days.

A student body assembly was held on May 20, where he said, “students expressed a deep sense of concern and compassion for the teachers, and were very angry with the students who committed these horrible acts.

“We ended this assembly with a prayerful reflection on the standards and expectations we have attempted to instill over the years in our community,” he said.